Alzheimer's disease (AD) is characterized by the presence of cerebral intraneuronal fibrillary tangles, extracellular neuritic plaques and amyloid fibril-laden vessels. The amyloid fibrils in these three pathological lesions are composed of an abundant amount of a highly insoluble 39-42 amino acid peptide called the beta-amyloid protein (betaAP) which is encoded by the amyloid precursor protein (APP) gene. The primary structure of the APP predicts a single transmembrane domain with a long N-terminal extracellular domain and a short cytoplasmic C- terminal domain. The betaAP domain is an internal peptide which starts in the extracellular domain 99 amino acids from the C-terminus residue and extends for 11-14 amino acids within the transmembrane domain. Proteolytic processing of the APP at amino acids 15-17 of the betaAP domain generates a secreted N-terminal peptide and a cellular C-terminal peptide therefore precluding the formation of the 39-42 amino acid amyloidogenic peptide. Processing of the amyloid precursor protein through the endosomal/lysosomal pathway generates a series of C-terminus peptides, some of which contain full length betaAP and are therefore potentially amyloidogenic. This pathway may be a key element in the formation of betaAP. However, since amyloid precursor protein is expressed in most cells but large amounts of betaAP occurs only in the CNS, there must be specific elements in the CNS which determine betaAP accumulation. In this proposal, we will study amyloid precursor protein post-translational proteolytic processing in the major cell types of the CNS such as astrocytes, neurons, and microglia from human cerebral cortex. Three specific aims are proposed: 1) To compare APP processing in the various cell types of the rat cerebral cortex as well as in meninges in order to identify which cell may have the most potential for the formation of betaAP and to compare with that of human non-AD and AD cerebral cortex to identify differences in processing between human and rat and between human non-AD and AD cerebral cortex. 2) To determine if a shift from the secretory pathway to the endosomal/lysosomal pathway leads to betaAP production. 3) To identify differences in APP processing with age in rat and human cerebral cortex.